Why MLP momentum term must be in the range 0-1?
If gradient descent is like walking down a slope, momentum would be the literal momentum of the agent traversing the hyperplane. 

Under that analogy then, momentum factor would be analogous to the friction coefficient, with 1 being max friction and 0 being no friction.

You should be able to see why there can't be friction beyond that range: if friction = 1 it would be identical to having no friction; if friction &lt;= 0 then by conservation of energy gradient descent will not find a local minima; if friction > 1 then gradient descent would be moving backwards.